Optical element

ABSTRACT

An element for protection against forgery and/or copying is arranged on a substrate and includes an optically anisotropic layer which has at least two regions with different optical axes, characterized in that the substrate is a reflective polarizer. The optically anisotropic layer can include cross-linked liquid crystal monomers. The anisotropic layer can be placed on an orientation layer. The orientation layer can be in contact with the polarizer and can include a photo-oriented polymer network. A device for protection against forgery and/or copying is characterized in that an element as set forth above and an analyzer are arranged on the same substrate.

This is a divisional of Ser. No. 14/017,352 filed Sep. 4, 2013, which isa divisional of application Ser. No. 12/692,435 filed Jan. 22, 2010, nowU.S. Pat. No. 8,553,196, which is a divisional of application Ser. No.11/931,937 filed Oct. 31, 2007, now U.S. Pat. No. 7,679,701, which is aDivisional of Ser. No. 11/455,747 filed Jun. 20, 2006, now U.S. Pat. No.7,292,292, which is a Continuation of application Ser. No. 10/728,853filed Dec. 8, 2003, now abandoned, which is a Divisional of applicationSer. No. 09/435,466, filed Nov. 8, 1999, now U.S. Pat. No. 6,734,936,which is a Continuation of PCT/IB98/00687 filed May 8, 1998, thedisclosures of all of which are hereby incorporated by reference.

The invention relates to an optical component containing an opticallyanisotropic layer, which layer has at least two regions with differentmolecular orientations. The anisotropic layer may, for example, be aretarder layer formed by cross-linked liquid-crystal monomers.

A particular use of the components according to the invention is in thefield of protection against forgery and copying.

The demand for safeguarding banknotes, credit cards, securities,identity cards and the like against forgery is increasing constantly onaccount of the high-quality copying techniques which are available.Furthermore, in low-wage countries, imitations of branded products andcopies of copyright-protected products, for example compact discs,computer software, electronics chips, etc. have been produced andexported worldwide. Because of the increasing number of forgeries, thereis therefore a great need for new elements which are safeguarded againstforgery and can be identified both visually and by machine.

In the field of copy-protecting banknotes, credit cards etc., there arealready a considerable number of authentication elements. Depending onthe value of the document to be protected, very simple or relativelyhighly complex elements are employed. Some countries are content toprovide banknotes with metal strips which come out black on a photocopy.Although this prevents them from being copied, elements of this type arevery easy to imitate. In contrast to this, there are also more complexauthentication elements, for example holograms and cinegrams.Authentication elements of this type are based on the diffraction oflight by gratings and need to be observed under different viewing anglesin order to verify their authenticity. These diffracted elements producethree-dimensional images, colour variations or kinematic effects whichdepend on the angle of observation and have to be checked on the basisof predetermined criteria or rules. It is not practically possible touse machines for reading information, for example images or numbers,encoded using this technique. Furthermore, the information content ofthese elements is very limited, and only an optical specialist will becapable of discriminating definitively between forgeries and anoriginal.

Lastly, one should not ignore the fact that diffractive optical effectshave in the course of time also been used outside the field of security,in particular for consumer articles such as wrapping paper, toys and thelike, and the production methods for such elements have in the course oftime become known to a large group of people and are correspondinglystraightforward to imitate.

Further to the diffractive elements mentioned above, other componentsare also known which are suitable for optimum copy protection. Theseinclude optical components, as disclosed for example by EP-A 689,684 orEP-A 689,065, that is to say components with an anisotropicliquid-crystal layer, which latter has local structuring of themolecular orientation.

These components are based on a hybrid layer structure which consists ofan orientation layer and a layer which is in contact with it andconsists of liquid-crystal monomers cross-linked with one another. Inthis case, the orientation layer consists of a photo-orientable polymernetwork (PPN)—synonymous with LPP used in ether literature which, in theoriented state, through a predetermined array, defines regions ofalternating orientations. During the production of the liquid-crystallayer structure, the liquid-crystal monomers are zonally orientedthrough interaction with the PPN layer. This orientation which, inparticular, is characterized by a spatially dependent variation of thedirection of the optical axis, is fixed by a subsequent cross-linkingstep, after which a cross-linked, optically structured liquid crystal(LCP or liquid crystal polymer) with a preestablished orientationpattern is formed. Under observation without additional aids, both theorientation pattern itself and the information written into the liquidcrystal before the liquid-crystal monomers are cross-linked, are atfirst invisible. The layers have a transparent appearance. If thesubstrate on which the layers are located transmits light, then the LCPorientation pattern or the information which has been written becomevisible if the optical element is placed between two polarizers. If thebirefringent liquid-crystal layer is located on a reflecting layer, thenthe pattern, or the corresponding information, can be made visible usingonly a single polarizer which is held over the element. LCPauthentication elements make it possible to store information, virtuallywithout restriction, in the form of text, images, photographs andcombinations thereof. In comparison with prior art authenticationelements, the LCP elements are distinguished in that the authenticity ofthe security feature can be verified even by a layman since it is notfirst necessary to learn how to recognise complicated colour changes orkinematic effects. Since LCP authentication elements are very simple,reliable and quick to read, machine-readable as well as visualinformation can be combined in the same authentication element.

As is likewise already known, the complexity of LCP authenticationelements can be increased further by inclining the optical axis of theLCP layer relative to the plane of the layer, uniformly or with localvariation. This can be done in known fashion by producing a PPN layerwith a locally varying tilt angle on the surface. This further providesa tilt effect, that is to say the information contained in thebirefringent layer is seen with positive or negative contrast dependingon the angle of observation. The object of the invention is now toprovide further possible layer structures of the above-mentioned typefor optical components, electro-optical devices and, in particular, forcopy protection elements.

According to the invention, this is achieved in that the physicalparameters and the configuration of the cross-linked liquid-crystallayer are varied and/or different layers, as well as a variety ofsubstrates, with differing respective optical properties are combined.Since the layers which are used are generally transparent, they can alsobe applied successfully to already known, permanently visibleauthentication elements, for example watermarks, holograms or cinegrams.The retarder pattern of the liquid-crystal layer can then be seen inaddition to the permanently visible authentication element onobservation using a linear polarizer.

When using the transmissive birefringent layers described in EP-A689,084, it is necessary to arrange one polarizer on each side of theelement in order to read or make visible the information which isstored. A quick check of identity cards and the like is in this casemade difficult by the involved positioning of the two polarizers aboveand below the authentication element. This disadvantage can be removedaccording to the invention by additionally integrating at least onepolarization layer in the layer structure. If there is, for example, apolarization layer below the birefringent layer, then one externalpolarization sheet, held over the element, is sufficient for making thestored optical information visible.

A polarization layer integrated in the authentication element can,according to EP-A 689,084, be designed as a dichroic LCP layer. It isalso possible to use a polarization sheet as a substrate for the PPN andLCP layers applied to it.

Where a reflector is present, which can be omitted according to thisinvention, the polariser sheet may possibly be the polariser for ingoinglight and the analyser for outgoing light, which may not always bedesirable.

A further disadvantage of the authentication elements described in EP-A689,084 is that, when arranging a polarizer below the substrate, thepolarization state of the light on passing through the substrate can beaffected. If, for example, use is made of inexpensive polymer substrateswhich, by virtue of the way in which they are produced, are themselvesbirefringent then since the birefringence of these substrates is arandom result of manufacture and varies from place to place, thebirefringence of the LCP layer may, in the extreme case, be cancelledout, with the result that the information of the authentication elementcan no longer be read. Furthermore, the use of strongly scatteringmaterials such as paper as a substrate is ruled out since polarizedlight would be immediately depolarized by these materials, so that thepolarization state of the light which passes through and is analyzedusing the second polarizer is unidentified and does not therefore carryany coded information.

However, if the integrated polarizer is, as proposed according to theinvention, located between the substrate and the LCP layer, then thesubstrate has no effect on the polarization state of the light onpassing through the LCP layer. As a result, on the one hand, it ispossible to use inexpensive polymer substrates which, by virtue of theway in which they are produced, are themselves birefringent, and on theother hand the substrate need not be transparent. In this case, evenscattering substrate materials can, for example paper and the like, arethus appropriate.

There are a variety of products, for example paintings, documents,photographs, compact discs, semiconductor chips, in which theauthentication element need not be visible since this would impair theoverall appearance of the product or would draw the attention of apotential product forger to the authentication element. For these cases,the invention proposes that orientable fluorescent dyes be incorporatedin a transmissive structured LCP layer.

There are yet further optical effects which can be used forliquid-crystal authentication elements. Examples include those producedby cholesteric filters. A known feature of these filters is that theyrefract, with circular polarization, a fraction of the visible lightspectrum in a wavelength range depending on physical parameters, whilethe unreflected light is transmitted (see: Schadt M., Fünfschilling J.,Jpn. J. Appl. Phys., 29 (1990) 1974). The effect of this is that thetransmitted light and the reflected light have different colours. Inorder for this to produce visual effects, it is necessary for thewavelength range of the selective reflection to lie in the visible lightrange. For applications in which the information is read by machine, itis of course possible for the refraction band to lie outside the visiblewavelength range.

Different types of optical components, which can likewise be used asauthentication elements in the field of copy protection, are based onthe combination of a linear polarizer, with a cholesteric filter. Aconfiguration of this type makes it possible (as also further explainedbelow) to produce different colours, for which use is in particular madeof a second linear polarizer arranged on the opposite side of thecholesteric filter from the first polarizer.

Lastly, the tilt effect described at the start can also be produced in adifferent way than is already known. It is thus possible, according tothe invention, to produce authentication elements whose tilt effects aremore pronounced and whose production is even simpler from a technicalpoint of view. This is achieved, in particular, in that at least onebirefringent LCP layer of an element is constructed in such a way thatits effective birefringence depends on the angle of observation. In thiscase, the optical axis may lie in the plane of the layer, i.e. it is notnecessary to incur the extra cost of tilting the optical axis out of theplane in a defined way.

According to the present invention, there is provided an opticalcomponent comprising at least two layers, characterized by a retarderand a polarizer, the retarder having at least two regions with differentoptical axes. Preferably the retarder comprises an anisotropic layercomprising cross-linked liquid-crystal monomers. The retarder may beplaced on an orientation layer and the orientation layer may be incontact with a polarizer. The orientation layer preferably comprises aphoto-oriented polymer network (PPN). The polariser may be placed on asubstrate. Optionally, a second polariser is arranged over theliquid-crystal layer and a further orientation layer and furtherliquid-crystal layer are arranged over this second polarizer, and thesecond liquid-crystal layer may also be structured. A further polarisermay be arranged over the second liquid-crystal layer, and a thirdorientation layer and a third liquid-crystal layer are arranged overthis further polarizer, and the third liquid-crystal layer may also bestructured. An element for protection against forgery and/or copying mayhave an optical component as set forth above and an external linear orcircular polarizer, the liquid-crystal layer encoding information whichcan be analyzed using the external polarizer. Such an element may becharacterised in that the at least two liquid-crystal layers each encodea partial information content which together form a total informationcontent. In this element, the liquid-crystal layer may be designed as aretarder and is preferably placed on a substrate characterized in thatthe substrate encodes a part of the total information content.Preferably, the external linear polarizer is structured, and both theliquid-crystal layer and the external polarizer each encode part of thetotal information content.

The optical component may be characterised by at least one circularpolarizer, or preferably by two circular polarizers arranged one abovethe other, one of which rotates to the left and the other of whichrotates to the right. An element for protection against forgery and/orcopying may contain such an optical component and an external linear orcircular polarizer for analysing the encoded information.

The invention also provides an optical component comprising an opticallyanisotropic layer which is formed by liquid-crystal molecules,characterised in that the optically anisotropic layer containsfluorescent molecules, and preferably has at least regions withdifferent optical axes. The invention extends to an element forprotection against forgery and/or copying including such an opticalcomponent.

The invention also provides an optical component comprising at least twolayers, characterized by a cholesteric layer and a linear polarizer andpreferably by an optically anisotropic layer, which may have regionswith different optical axes. The optically anisotropic layer may beformed of cross-linked liquid crystal molecules. The cholesteric layerand the optically anisotropic layer are preferably on the same side ofthe linear polariser, which may be in contact with the cholestericlayer. The linear polariser may be arranged on a substrate, thecholesteric layer being in contact with the linear polariser, and anorientation layer may be placed on the cholesteric layer, and anoptically anisotropic layer of cross-linked liquid-crystal monomers maybe placed on the orientation layer, the liquid crystal (opticallyanisotropic) layer forming regions with different molecularorientations. An element for protection against forgery and/or copyinghave such an optical component and an external linear polarizer foranalysing the information encoded in the liquid-crystal layer and/or inthe cholesteric layer.

The invention also provides an optical component, containing abirefringent liquid-crystal layer which has at least two regions withdifferent optical axes, characterized in that the optical delay of theliquid-crystal layer in the individual regions depends differently onthe angle of observation. This component may be designed in such a waythat the colour of the element on observation through a polarizerdiffers locally, and may be biaxial; preferably the birefringent layeris biaxial. An element for protection against forgery and/or copying mayhave such an optical component. A further element, according to theinvention, for protection against forgery and/or copying comprises apolariser layer which has at least two regions with differentpolarisation directions.

A further such element is arranged on a substrate and comprises anoptically anisotropic layer which has at least two regions withdifferent optical axes, the substrate being a reflective polariser.

The invention also provides a device for protection against forgeryand/or copying, wherein an element of any of the types set forth aboveand an analyser are arranged on the same substrate, such as acertificate or banknote.

Some of these may be considered as documents carrying invisible proof ofauthenticity, often in polarised light form. Some such documents,lacking a reflective layer, may be authenticable using illumination fromunderneath (transmitted through the document to the viewer). Some suchdocuments may advantageously lack an integrated polariser.

Illustrative embodiments of the invention will now be described belowwith reference to the appended drawing. In a simplified schematicrepresentation,

FIG. 1 shows a layer structure of an optical component consisting of apolarizer, a PPN layer and an LCP layer, as well as the associatedanalyzer,

FIG. 2 shows the LCP structuring of the component in FIG. 1,

FIG. 3 shows a foldable document with an element of the typecharacterized in FIG. 1,

FIG. 4 shows a layer structure constructed in an alternative way to thestructure in FIG. 1, with an additional PPN layer and LCP layer, as wellas an analyzer which is arranged after the layer structure in thedirection in which light travels,

FIG. 5 shows a layer structure constructed in an alternative way to thestructure in FIG. 1, with an additional PPN layer and LCP layer, as wellas an analyzer which is arranged before the layer structure in thedirection in which light travels,

FIG. 6 shows a layer structure constructed in an alternative way to thestructure in FIG. 1, with two additional PPN and LCP layers, as well astwo external polarizers on opposite sides,

FIGS. 7 a and 7 b show an LCP component which has a locally differentorientation and a cholesteric filter, as well as a polarizer arrangedafter the element in the direction in which light travels,

FIGS. 8 a and 8 b show a layer structure which is of the type shown inFIG. 7, but with a polarizer arranged before the element in thedirection in which light travels,

FIGS. 9 a and 9 b show a layer structure which is of the type shown inFIG. 7 but with an additional cholesteric filter,

FIGS. 10 a and 10 b show a layer structure which is of the type shown inFIG. 7, but in which the cholesteric filter and the polarizer areinterchanged,

FIG. 11 shows a two-layer authentication element consisting of acholesteric filter and a first linear polarizer, as well as anassociated analyzer, and

FIG. 12 shows a layer structure as in FIG. 11, but with an additionalretarder layer.

The schematic section represented in FIG. 1 through a layer structure ofa first illustrative embodiment according to the invention shows asubstrate 1, made of a transparent material such as glass, for example,or of a scattering material, such as paper, for example. On thesubstrate, there is a linear polarizer 2, on which there is a layer 3 ofa photo-oriented polymer network (PPN) whose orientation varies locally(e.g. imagewise) over its surface on the substrate. Examples ofmaterials which are suitable for this include cinnamic acid derivatives,as described for example in EP-A 525,478 or U.S. Pat. No. 5,389,698.They are oriented and at the same time cross-linked by selectiveexposure to linearly polarized UV light.

An anisotropic layer 4 of cross-linked liquid-crystal monomers adjoinsthe layer 3. This LCP layer 4 consists in this case of a moleculararrangement whose orientation is predetermined by the orientation of theunderlying layer 2. Using light of a suitable wavelength, the LCP layer4 is photo-cross-linked, by means of which the molecular orientationdefined by the PPN layer 3 is fixed. Using an external polarizer 5, theorientation pattern or the stored optical information (i.e. the image)can be made visible, for which purpose light passes from below in thedirection of the arrow 6 through the element denoted overall as 7, andthe polarizer 5 (acting in this case as an analyzer) is held over theelement 7.

FIG. 2 shows the preferred mutual orientation of the optical axes ofadjacent locally structured regions of the LCP layer 4. In order toproduce maximum contrast, the optical axes of adjacent regions areangled at 45□.

FIG. 3 shows a variant, still according to the invention for simplifyingthe verification of such LCP security items. In this case, the second(external) polarizer 5 is mounted on a light-transmitting flexiblesubstrate 8, such as a document or a banknote. This is done in such away that the polarizer 5 can be positioned over the element 7 mountedelsewhere on the same banknote, by folding or bending the banknote 8, sothat the stored LCP image can be seen through the polarizer 5 on lookingthrough. In this way, both polarizers needed for recognizing the storedauthentication element are present on the same substrate, with theresult that it is not necessary to have external polarizers, and thus nofurther aids for analysing the information are necessary.

Of course, the second polarizer 5 may itself again form part of a layerstructure which in turn bears an LCP layer. On the one hand, there arethen simultaneously two LCP layer structures on one substrate, withinformation content which can be made visible separately from oneanother as individual patterns, in each case using an externalpolarizer. On the other hand, the optical anisotropies of the two LCPlayers can also be combined with one another if the substrate is bentand then viewed through two polarizers. In this case, a third pattern isproduced which differs from the two individual patterns.

Complexity, surprise, and the optical quality and information contentcan all be increased according to the invention by making the layerstructure of two information-carrying LCP layers sandwiching apolarization layer. Depending on whether a second, external polarizer isthen arranged above or below the layer structure, one or other of theinformation contents can be seen. The arrangement of the layers ofcorresponding elements are represented schematically in FIGS. 4 and 5.In this case, the two PPN and LCP layers, respectively together forminga pair, are denoted 11 a and 11 b or 12 a and 12 b, respectively, andthe polarizer layer arranged between the two pairs is denoted 13. Theexternal polarizer which acts as an analyzer is here denoted 14 a or 14b, and the direction of the light is denoted 15.

If, however, one external polarizer, notionally 14 a and 14 b (notshown), is arranged both above and below, then both information contentsare seen at the same time. If one or both external polarizers arerotated through 90°, the information contents will be invertedindependently of one another, that is to say represented in negative.For example, an image could be stored in one of the two LCP layers andcorresponding textual information could be written in the other. Bychoosing the arrangement of the polarizer, it is then possible to makeonly the image or only the text visible, or bath visible at the sametime.

Analogously with the examples described above with reference to FIGS. 1to 5, the number of PPN and LCP layers can be increased further. In thecase of an element 29 having a three-layer structure (FIG. 6), thelayers 21 a/21 b, 22 a/22 b and 23 a/23 b are separated from one anotherby two crossed polarizer layers 24 and 25. In this layer structure, thecentral LCP layer 22 b arranged between the two polarizers 24 and 25 canbe produced according to the method disclosed in EP-A 689,084. Withlight 26 incident at right angles to the plane of the layer, theinformation in the central layer 22 b is in this case permanentlyvisible, while the information in the upper or lower LCP layers 23 b and21 b, respectively, can as described above be made visible by arrangingan external polarizer 27 or 28 above or below the element 29. If boththe polarizer 27 and the polarizer 28 are applied simultaneously to eachside of the element 29, then the information in all three LCP layers canbe made visible at the same time. In this way, for example, a singleimage can be broken down and distributed between the three LCP layers 21b, 22 b and 23 b. Only by arranging one or two external polarizers willthe individual parts of the image be recombined to form the originalimage.

The information in the central LCP layer may, however, also be codedthrough locally varying tilt angle, or through tilt effects of the typewhich will be described below, that is to say, for example, throughspatially varying directions of the optical axis relative to the planeof the layer. The result of this, in the case of the layer systemconsisting of three LCP layers with polarization layers lying inbetween, is that the information in the central image cannot initiallybe seen so long as the layer is viewed at right angles. Only onobservation at an oblique angle does the information in the centrallayer become visible, because of the different birefringence of regionswith different tilt directions for the optical axis. By using one or twoexternal polarizers, the information contents of the lower and/or upperlayers are then visible at the same time as the information in thecentral layer.

The complexity can be increased by further LCP layers which arerespectively separated from the others by polarization layers. Theinformation in each of the LCP layers can thus be stored differently,for example through local variation of the direction of the optical axisin the plane as well as out of the plane. As a result, the informationcontent in the individual layers can be viewed independently of eachother according to the angle of observation and the arrangement ofexternal polarizers.

Linearly polarizing layers can also be produced using LCP layers whichcontain dichroic dye molecules. The dichroic molecules orient in suchlayers according to the local orientation of the LCP molecules, so thatlight is linearly polarized locally in the layer, that is to sayaccording to the orientation of the dichroic dye molecules. Bystructuring the doped LCP layer, it is thereby possible to producepolarization layers with locally differing polarization direction. Thebrightness and/or colour of the birefringent layer between twopolarizers depends on the direction of the optical axis of the retarderlayer, as well as on the transmission directions of the two polarizers,one (or both) of the polarizers needed to visualize the retarder patterncan themselves be structured and therefore carry information. Thepatterns in the retarders and polarizers can then be matched with oneanother. It is thus possible to put one part of the information in theLCP layer and another part in the polarization layer. The totalinformation content can then be read only by an individual who isprovided with the structured polarizer matching the retarder layer. Ifthere is a reflector under the structured retarder layer, then thesecond (optionally unstructured) polarizer underneath the retarder layeris no longer required for reading the information. However, just as partof the information content can be put in the analyzer, part of theinformation may already be present permanently on the substrate. In thisway, for example, a photograph can be broken down into a partpermanently visible on the substrate, and an initially invisible partwhich is put in the retarder layer and cannot be seen unless a polarizeris used. In the case of a LCP pattern on a reflector, a further variantcould be the reflector itself structured. On observation through apolarizer, the additional information which is stored in the structuredretarder layer is then seen inside the reflecting regions.

As already mentioned at the start, there a variety of products, forexample paintings, documents, photographs, compact discs andsemiconductor chips, in which the authentication element is not intendedto be visible. Transmissive structured retarder layers would satisfythis condition, but in order to visualize the information which theycontain, a polarizer is placed before and after the retarder layer,which is possible only if the substrate does not alter the polarizationstate of the light. In contrast, in the case of reflective elementsbased on structured retarders, it is necessary for there to be areflector, which as a rule can always be seen, under the retarder layer.

For cases of this type, it is a further object to provide anauthentication element which although carrying retrievable information,cannot be seen under normal conditions. According to the invention, thisis achieved in that the orientable fluorescent dyes, which eitherfluoresce anisotropically or (and) absorb light anisotropically and haveabsorption bands in the UV range, are incorporated in a structured LCPlayer. If the fluorescent molecules are chosen suitably, than onexposure to polarized UV light, those molecules whose transition momentis parallel to the polarization direction of the exciting UV light, arepreferably excited. In an LCP layer in which the fluorescent moleculesare zonally perpendicular to one another in accordance with the LCPorientation, only those regions whose orientation is parallel to thepolarization direction of the UV light will consequently fluoresce, andthis makes it possible to see the information stored in the layer whichis invisible in the absence of UV excitation.

As an alternative, the doped LCP layer may also be excited withisotropic light. If the fluorescent molecules are chosen suitably, theyradiate the fluorescent light with a polarization, the direction of thepolarization being determined by the orientation of the molecules. Usinga polarizer, it is possible to discriminate between regions withdifferent polarization of fluorescent light, and this makes it possibleto see the information present in the layer.

FIGS. 7 to 10 show optical elements with at least one cholesteric which,as already mentioned at the start, can also be used for authenticationelements with cross-linked liquid-crystal molecules.

In a first illustrative embodiment (FIGS. 7 a and 7 b) of this categoryof elements, use is made of a structured LCP retarder layer 31 whoseoptical delay or path difference is λ/4, and in which the information isencoded by means of regions whose optical axes are perpendicular to oneanother. If a cholesteric filter 33 whose selective reflectionwavelength λ_(R) lies in the visible light range is then placed underthis structured retarder layer 31, or under its PPN orientation layer32, then light passing through the cholesteric filter in the directionof the arrow 34 from below, will first be circularly polarized in theregion of the selective bands. On passing through the structure retarderlayer 31, the circularly polarized light will then be converted intolinearly polarized light because of the λ_(R)/4 optical delay. Since, asrepresented in FIG. 7 b, the optical axes in the differently orientedregions are perpendicular to one another, the polarization direction ofthe linearly polarized light after passing through the correspondingregions is also rotated through 90° relative to one another. If a linearpolarizer 35 having a transmission angle β=45°, measured relative to thedirections of the mutually perpendicular optical axes of the retarderlayer 31, is then held over this arrangement, then coloured andcolourless regions will be seen. When the polarization 35 is rotatedthrough 90°, the optical properties of the regions will be interchanged.

On the other hand, if the light is not put into the element through thearrangement consisting of the filter 33, the PPN layer 32 and theretarder layer 31, but is incident through the linear polarizer fromabove, as shown in FIG. 8, then the pattern which has been written willbe seen in complementary colours in the reflective light. In this way,it is possible to produce authentication elements with high informationcontent, in which the information appears as complementary coloursdepending on whether the transmitted or reflected light is observed.

Both the circular polarizer and the linear polarizer may form part ofthe layer structure, in which case they are permanently present. Theymay, however, be arranged above or below the layer only when theinformation is read. A circular polarizer layer may, for example, beformed from a layer of chiral LCP material which is only a fewmicrometres thick.

The element represented in FIG. 9 a has a similar design to the elementin FIG. 7, and has a structured retarder layer 41 with an optical delayof about λ/4. In this case as well, the information is encoded by meansof regions with mutually perpendicular optical axes, as shown in FIG. 9b. In this illustrative embodiment, however, one left-rotating and oneright-rotating cholesteric filter 42 and 43, respectively, are arrangedin series under the PPN layer 44 belonging to the retarder layer 41. Themaxima of the selected reflection bands of the two filters 42 and 43 liein different wavelength ranges. If a linear polarizer 45 is again heldover the structured retarder layer, then the regions with mutuallyperpendicular optical axes appear with different colours. When thepolarizer or the retarder layer is rotated through 90□, the colours ofthe regions are interchanged.

A final further element in this category is shown by FIGS. 10 a and 10b. In this case as well, use is made of a structured λ/4 retarder layer51, in which the information is encoded by means of regions withmutually perpendicular optical axes. In contrast to the example in FIG.7, the linear polarizer 52 and the cholesteric filter 53 are in thiscase interchanged. Light incident form below in the direction of thearrow 54 will firstly undergo uniform linear polarization by the linearpolarizer 52 and, on passing through the structured retarder layer 51,will become left or right circularly polarized depending on thedirection of the optical axis. If a cholesteric filter 53 which acts asa circular polarizer is held above, then either left or right circularlypolarized light will be transmitted, depending on the sense of rotationof the circular polarizer, and light with the opposite sense of rotationwill be reflected. The pattern written in the retarder layer 51, encodedby different optical axis directions, then appears as a pattern ofbright coloured regions.

If, in this special case, the circular polarizer 53 is replaced by asecond linear polarizer, then the pattern cannot be seen since thepolarization state of the light after passing through the regions of theretarder layer 51 is either right or left circularly polarized.

The fact that retarder regions whose optical axes are mutuallyperpendicular cannot be distinguished using linear polarizers, opens upthe possibility of writing different information contents in an LCPlayer, it being possible for these to be read independently of oneanother using different aids. To do this, for example, firstinformation, as described in the illustrative embodiment in FIG. 10, canbe encoded using regions with mutually perpendicular optical axes.Second information is then encoded using regions whose optical axes makean angle of 45° with the mutually perpendicular axes in the said firstregions. If, as described in the illustrative embodiment in FIG. 10, alinear polarizer is placed under the retarder layer and the layer isilluminated through it, then only the second information is seen whenusing a second linear polarizer which is held over the element formed bythe linear polarizer, PPN layer and LCP layer. In contrast, the firstinformation is seen with a normal observation angle, only if (as alreadyexplained) a circular polarizer instead of the linear polarizer is heldover the retarder layer, and in this case the second information canalso be seen with a reduced intensity. In an authentication element, itwould thus, for example, be possible to have a polarization layerpermanently integrated under the structured retarder layer, so that inorder to verify the authenticity of the element, it is sufficient tohold the linear polarizer and circular polarizer successively over thesaid element in order to see the different information contents.

Lastly, it will be pointed in this regard that, when at least onecholesteric filter is used, there is the further possibility, in orderto visualize a retarder structure, of not using any linear polarizers,but only using circular polarizers. The information is, for thispurpose, recorded by structuring the optical delay in the retarderlayer, it then being possible for the optical axis to have the samedirection throughout the plane of the layer. If a retarder layer of thistype is placed between two cholesteric filters whose selectivereflection bands overlap, then the information which is written will bevisible or readable.

As already mentioned at the start, there is a further possibility ofdeveloping optical authentication elements which are essentially formedby a cholesteric filter and two linear polarizers.

This is because combining a linear polarizer with a cholesteric filtermakes it possible to produce different colours if a second linearpolarizer, used as an analyzer, is arranged on the opposite side of thecholesteric filter from the first polarizer.

In the simplest case, an authentication element employing this effectwould consist only of one cholesteric layer. In order to produce anoptical effect usable for authentication elements, it is then necessaryto have two linear polarizers which, as required, are to be held over orunder the cholesteric layer. In this simple case, the cholesteric layermay only be applied to a transparent substrate, for example glass.However, if the authentication element is to be applied to a diffusedepolarizing substrate, then the first polarizer may be integratedpermanently in the authentication element. An authentication element ofthis type is represented in FIG. 11. This element consists of acholesteric layer 61, a substrate 62 and a first polarizer 63, arrangedbetween the substrate 62 and the layer 61. The second linear polarizer,required for observing the stared information, is denoted 64 and, whenrequired, should be held over the said element.

The colour of the light passing in the direction of the arrow 65 throughthe linear polarizer 63 in the cholesteric filter 61 is firstlydetermined by the wavelength of the selective reflection of thecholesteric filter 61. If the external linear polarizer 64 is then heldover the cholesteric layer, then the colour changes when the polarizer64 is rotated. If, for example, use is made of a cholesteric filter 61which reflects the colour green, then it firstly appears red-violet intransmission. Conversely, if the layer is observed through the secondpolarizer 64, then on rotation of this polarizer the colours yellow,green, red or blue are seen.

If a uniaxial optical delay layer with a path difference of for example,λ/2, is then placed between the cholesteric filter 61 and the secondpolarizer 64, then for a constant position of the polarizer 64, thecolours are changed by rotating the delay layer. Through suitable choiceof reflection wavelength and bandwidth for the cholesteric filter, andthrough suitable choice of the optical path difference and the directionof the optical axis of the delay layer, it is in this way possible toproduce a broad palette of colours. Instead of between the cholestericfilter 61 and the polarizer 64, the delay layer may also be locatedbetween the input polarizer 63 and the cholesteric filter 61.

So long as an unstructured delay layer is used, the colour effects donot differ very greatly from those achieved using a single cholestericlayer between two polarizers. However when use is made of structuredretarder layers in which the optical axis zonally has a differentalignment, it is possible to produce locally different colours. Oneembodiment of an authentication element designed in this way isrepresented in FIG. 12. It consists of a first polarizer 72, placed on asubstrate 71, a cholesteric layer 73 and a structured LCP retarder layer74 with an associated PPN orientation layer 75. If this element isplaced under an external polarizer 76 whose polarization direction is,for example, perpendicular to the polarization direction of thepolarizer 72, then different colours are seen, the number of differentcolours depending on the structuring of the retarder layer 74 and beingdetermined by the number of differently aligned optical axes. In thisway, information can be represented in colour. This impressive opticaleffect is further enhanced in that the respective colours change whenthe polarizer 76 is rotated. Furthermore, on account of the dependenceof these selective reflection wavelengths on viewing angle, and onaccount of the optical path difference in the retarder layer, anauthentication element of this type has a pronounced colour dependenceon observation angle.

Further to structuring the direction of the optical axis, it is alsopossible to structure the optical delay in the retarder layer. It isthereby possible to optimize the appearance of colour using anadditional parameter.

Although the combination of cholesteric filter and optical delay layermakes it possible to represent a large number of colours, it isnevertheless not possible with this arrangement to adjust the brightnessof the colours over the full range from dark to bright. This can,however, be achieved by structuring the cholesteric filter, for exampleby locally removing the cholesteric layer by photolithography, or byshifting the reflection wavelength of the cholesteric filter when it isbeing produced by locally varying the path length in the invisiblewavelength range. Since the cholesteric filter is not present, or isoptically isotropic, at points treated in this way, only the retarderlayer determines the optical behaviour at these points. In the case ofcrossed polarizers, it is for example possible for the optical axis tobe set parallel to one of the polarizers, as a result of which the lightat this point is blocked and therefore appears dark. By varying theratio of the areas of dark and coloured regions, it is thus possible tocontrol the brightness of the individual colours (mosaic picture).

As already mentioned above, the tilt effect described at the start inbirefringent layers can also be produced in a different way than isalready known, by means of which it is possible to make authenticationelements whose tilt effect is more pronounced and whose production iseven easier to carry out.

According to the invention, this is achieved in that at least onebirefringent layer of the layer structure is constructed in such a waythat its effective optical delay depends on the angle of observation. inthis case, the optical axis may lie in the plane of the layer, i.e.there is no need to pay the extra cost of tilting the optical axis outof the plane in defined fashion. The optical delay is equal to theproduct of the layer thickness and the optical anisotropy of thematerial, so that the optical delay for a given material can be adjustedby means of the layer thickness. Depending on the value of the opticaldelay, the layer appears with different colours or grey on observationusing crossed polarisers. If the effect of optical delay is thendependent on the angle of observation, then the grey value or the colourchanges correspondingly with the angle of observation. For example, witha material having positive, uniaxial optical anisotropy, the opticaldelay can be adjusted in such a way that the layer appears violet whenobserved vertically. If however, the layer is viewed obliquely, in sucha way that the viewing angle and the optical axis form a plane, then thecolour changes from violet to yellow. If, however, one looks obliquelyfrom a direction which is perpendicular to the optical axis, then thecolour changes from violet to blue. With a corresponding position of theoptical axis, it is thus possible to achieve the effect that, when thelayer is tilted downward or upward, the colour changes from violet toyellow, while it changes to blue when the layer is tilted to the rightor to the left.

This angular dependence of the optical delay can be employed to producestructured LCP authentication elements with information written in them,this information having an angle-dependent appearance. If, for example,an LCP layer is structured as described in EP-A-689084, in such awaythat the optical axis of different regions are, in accordance with theinformation to be represented, either parallel or perpendicular to areference axis lying in the plane of the layer, then the informationcannot at first be seen under vertical observation with crossedpolarizers. Only when the layer is observed obliquely does it becomepossible to see the pattern which has been written, since the angle ofobservation is then different for regions whose optical axes areperpendicular to one another. If the layer thickness is then againadjusted in such a way that the optical delay appears violet under thevertical observation, then the colour changes from violet to blue ontilting about the reference axis in those regions where the optical axisis parallel to the reference axis, while the colour in the other regionssimultaneously changes from violet to yellow. If the layer is tiltedupward or downward, then the information thus appears yellow on a bluebackground or blue on a yellow background if the layer is tilted to theleft or to the right. Of course, by means of the layer thickness it islikewise possible to set other colours, grey values or combinations ofcolours with grey values. When grey values are used, a black and whiteeffect is obtained instead of the colour effect.

In order to produce birefringent layers whose apparent image changeswith the viewing angle, both uniaxially and biaxially birefringentmaterials are suitable. However, the dependence on viewing angle can beenhanced further by using optically biaxial materials. If, for example,the refractive index perpendicular to the plane of the layer is lessthan the refractive index in the plane of the layer, then the opticaldelay and therefore the tilt effect under oblique observation changemuch more than in the case of a uniaxial material.

Instead of using a biaxial material, the strong dependence of viewingangle can also be achieved by a layer structure made of two or moreuniaxial layers, the optical axis in one layer being, for example,parallel or oblique with respect to the plane of the layer, while in asecond layer it is perpendicular to the plane of the layer. Throughsuitable choice of the ratio between the layer thicknesses, the tilteffect can be made more intense or weaker. If, furthermore, the layer inwhich the optical axis is parallel or oblique with respect to the planeof the layer is also structured, that is to say the projection of theoptical axis onto the plane of the layer points zonally in differentazimuthal directions, then under crossed polarizers with obliqueobservation, a pattern is seen whose colours or grey values change withgreat effect when the observation angle is altered only slightly.

In a further illustrative embodiment, the strong dependence on viewingangle can also be achieved by a layer structure which contains anunstructured optically biaxial layer as well as a structuredbirefringent layer of optically uniaxial material. This can, forexample, be brought about very simply by applying the structuredbirefringent layer directly onto an optically biaxial sheet.

Authentication elements having a dependence on viewing angle can also bemade by using a substrate which can polarize incident light as afunction of angle. This is, for example, the case with non-metallicsmooth surfaces, for example glass or plastic. Obliquely incident lightwhich is reflected from the surface of such materials is at least partlypolarized. Under a particular angle of incidence (the Brewster angle),which depends on the respective material, the reflected light is in factcompletely linearly polarized. If use is made of such a material withangle-dependent polarizing effect as a substrate for structured retarderlayers, then obliquely incident light which is reflected from thesurface of the substrate will be polarized before it passes againthrough the retarder layer. The polarization state is then changed as afunction of the local direction of the optical axis, so that a patterncan be seen in a correspondingly structured retarder layer if a layer ofthis type is viewed obliquely through a polarizer. The optimum contrastis achieved if the layer is viewed at the Brewster angle. The patterndisappears completely when the angle of observation is normal.

Instead of birefringent layers, it is also possible to produce tilteffects by using layers which anisotropically absorb light. Layers ofthis type can, for example, be made with LCP layers in which dichroicdyes are incorporated. Since the dichroic dyes are oriented with the LCPmolecules, the dichroic dyes can likewise be given a zonally differentorientation through structured orientation of the LCP molecules. Onpassing through the layer, originally isotropic light then becomeslinearly polarized, the polarization direction being locally differentand determined by the local orientation of the LCP or dichroicmolecules. Depending on the dye which is used, it is possible topolarize light within the visible range or only within a singlewavelength range, so that the layers appear either grey or coloured. Thepattern which is written can be seen if the layer is observed through alinear polarizer.

LCP layers which contain dichroic dyes exhibit absorption which dependson the viewing angle. If a uniaxially oriented LCP layer which is dopedwith dichroic dyes is tilted about the orientation direction of the LCPor dye molecules, then because of the increase in the optical path withincreasing tilt angle, the layer appears darker than with a normal angleof observation. However, if the layer is tilted about an axis lyingperpendicular to the LCP orientation direction in the plane of thelayer, then the layer appears brighter since the absorption axis of thedye molecules is in this case oblique with respect to the incidencedirection of the light, which has the result that a smaller proportionof the light is absorbed. In order to see these variations in brightnessdue to tilting, it is not absolutely necessary to observe the layerthrough a polarizer. If, for example, an LCP layer is then structured insuch a way that, in different regions, the LCP molecule are parallel orperpendicular to one another, then when the layer is tilted about one ofthese two preferential directions, those regions with the LCPorientation parallel to the tilt axis appear darker, while the othersappear brighter. Conversely, if the layer is tilted about the otherpreferential axis, then the brightness of the regions is interchanged.It is possible to see this effect as well without using an additionalpolarizer, and it is therefore particularly suitable for applicationswhere the intention is to check an authentication element without anadditional aid.

The production of a PPN and LCP layer which can be used according to theinvention, as well as the production clan authentication element with atilt effect, will be explained in more detail below.

1. Production of a PPN Layer

Suitable PPN materials include, for example, cinnamic acid derivatives.For the investigations fundamental to the present invention, a PPNmaterial with high glass point (T_(g)=133° C.) was chosen:

Polymer:

A glass plate was spin-coated with a 5 percent strength solution of thePPN material in NMP for one minute at 2000 rpm. The layer was then driedfor one hour at 130□C on a heating bench and for a further hour in avacuum. The layer was then exposed to linearly polarized light, 200 W Hghigh-pressure lamp for 5 minutes at room temperature. The layer was thenused as an orientation layer for liquid crystals.

2. Mixture of Cross-Linkable LC Monomers for the LCP Layer.

In the examples, the following diacrylate components were used ascross-linkable LC monomers.

Using these components, a supercoolable nematic mixture M_(LCP) withparticularly low melting point (TM≈35° C.) was developed, making itpossible to prepare the LCP layer at room temperature.

The diacrylate monomers were present with the following composition inthe mixture:

Mon1 80% Mon2 15% Mon3  5%

In addition a further 2% of the Ciba-Geigy photoinitiator IRGACURE 359were added to the mixture.

The mixture M_(LCP) was then dissolved in anisol. By means of theM_(LCP) concentration in anisol, it was possible to adjust the LCP layerthickness over a wide range.

For photoinitiated cross-linking of the LC monomers, the layers wereexposed to isotropic light from a 150 W xenon lamp for about 30 minutesin an inert atmosphere.

3. Authentication Element with Tilt Effect

The two halves of a PPN-coated glass plate were exposed to polarized UVlight, the polarization direction of the light when illuminating thesecond half being rotated through 90° relative to the first exposure. Ineach case, the other half was covered during the exposure. This gave tworegions with planar, mutually perpendicular orientation directions.

A 5 percent strength solution of M_(LCP) in anisol was produced. Thesolution was spun onto the PPN layer that had been exposed in thedifferent ways. Spin parameters: 2 minutes at 1000 rpm. In order tooptimize the orientation of the LC monomers, a coated substrate was thenheated to just above the clearing point (T_(C)=67° C.). The layer wasthen cooled at a rate of 0.1° C./min to three degrees below the clearingpoint.

After the LC monomers had cross-linked, the thickness of the LCP layerwhich was obtained was about 80 nm.

If this layer is arranged between crossed polarizers in such a way thatthe orientation directions of the LCP layer form an angle of 45° withthe transmission directions of the polarizers, then the LCP layerappears uniformly grey. If, however, the layer is observed obliquely,with the viewing direction and the orientation direction of theleft-hand half of the plate forming a plane, then the left-hand half ofthe plate appears darker while the right-hand half of the plate appearslighter.

To conclude, it should be pointed out that the optical effects describedabove, as well as the corresponding layer structures and materialcompositions, represent no more than a choice from a plurality ofembodiments according to the invention, and may in particular becombined its a wide variety of ways in order to develop authenticatingelements.

Thus, it is of course possible for any other kind of birefringent layerusing which it is possible to produce an optical effect that can beemployed, for example for authentication elements, to be put into theoptical component instead of an LCP layer.

It is furthermore possible for the examples described above, instead ofa PPN orientation layer, to use a different orientation layer which,according to the desired optical property and resolution, has the sameor similar properties to a PPN layer. It is also conceivable to producethe orientation required for a retarder layer using a correspondinglystructured substrate. A structured substrate of this type can, forexample, be produced by embossing, etching and scratching.

Lastly, it should be pointed out that the multilayer structuresaccording to the invention can be used not only as elements forsafeguarding against forgery and copying, but for example can also beused to produce electro-optical liquid-crystal cells in which the LCPlayer fulfils various optical and orienting functions.

What is claimed is:
 1. Element for protection against forgery and/orcopying, being arranged on a substrate and comprising an opticallyanisotropic layer which has at least two regions with different opticalaxes, characterized in that the substrate is a reflective polarizer. 2.Element according to claim 1, wherein the optically anisotropic layercomprises cross-linked liquid crystal monomers.
 3. Element according toclaim 2, wherein the anisotropic layer is placed on an orientationlayer.
 4. Element according to claim 3, wherein the orientation layer isin contact with the polarizer.
 5. Element according to claim 3, whereinthe orientation layer comprises a photo-oriented polymer network. 6.Device for protection against forgery and/or copying, characterized inthat an element according to claim 1 and an analyzer are arranged on thesame substrate.
 7. Device for protection against forgery and/or copying,characterized in that an element according to claim 2 and an analyzerare arranged on the same substrate.
 8. Device for protection againstforgery and/or copying, characterized in that an element according toclaim 3 and an analyzer are arranged on the same substrate.
 9. Devicefor protection against forgery and/or copying, characterized in that anelement according to claim 4 and an analyzer are arranged on the samesubstrate.
 10. Device for protection against forgery and/or copying,characterized in that an element according to claim 5 and an analyzerare arranged on the same substrate.